Plastic Water Absorption Testing and Analysis: Standards, Methods, and Technical Interpretation

Plastic water absorption testing quantifies the amount of moisture a polymer material takes up under controlled immersion or humidity conditions, providing essential data on dimensional stability, mechanical strength, and electrical insulation performance. Governed by internationally recognized standards such as ISO 62 and ASTM D570, this analysis supports material qualification, quality control, and design decisions across the automotive, electronics, packaging, and construction industries. This article explains the testing principles, sample requirements, test methods, calculation procedures, and practical applications of water absorption determination for plastic materials.

Technical Overview

Water absorption refers to the uptake of water molecules by a plastic material when it is exposed to liquid water or a humid atmosphere. Because absorbed water behaves as a plasticizer within the polymer matrix, even small amounts of moisture can significantly reduce mechanical strength and elastic modulus, lower the glass transition temperature, degrade dielectric and insulation properties, and cause dimensional swelling or warpage in finished components.

Water penetrates plastics primarily through molecular diffusion into the free volume of the polymer structure, and in polar polymers it also forms hydrogen bonds with functional groups along the polymer chains. Amorphous regions permit diffusion far more readily than crystalline regions; therefore, highly crystalline or non-polar polymers such as polypropylene, polyethylene, and PTFE exhibit very low absorption, while polar polymers such as polyamide (nylon) and cellulose-based plastics can absorb substantial amounts of moisture.

The principal reference standards for this analysis include ISO 62 (Plastics — Determination of water absorption), ASTM D570 (Standard Test Method for Water Absorption of Plastics), and GB/T 1034, the Chinese national standard that is technically harmonized with ISO 62. JIS K 7209 provides a corresponding Japanese procedure. These standards define specimen geometry, conditioning requirements, exposure conditions, and the calculation method used to express results as a percentage of the dry mass.

Testing Samples

Standard Molded Specimens

The preferred specimen for compression or injection molded materials is a disc of 50 mm to 60 mm diameter and 3 mm thickness. Using a standardized geometry ensures consistent surface-to-volume ratio, which is critical because water absorption is a diffusion-controlled, thickness-dependent process. Specimens must be free from voids, sink marks, flash, and surface contamination that could distort the mass change measurement.

Sheet and Film Materials

Square specimens of approximately 50 mm × 50 mm are typically cut from sheets or films using a sharp die or blade to avoid edge damage. Because thin films reach saturation much faster than molded discs, the exposure schedule may be adjusted while the reporting basis (percentage of dry mass) remains unchanged.

Rods, Tubes, and Pipes

Cylindrical products are tested as segments, commonly 50 mm in length. Depending on the engineering intent, the end faces may be left exposed to evaluate total absorption through all surfaces or sealed with a stable coating so that only radial diffusion through the wall is measured.

Specimens from Finished Products

When testing is performed for product qualification or failure analysis, specimens may be cut directly from actual components. In these cases, the exact dimensions and mass are recorded, and results are frequently reported as mass change per unit surface area (mg/cm²) rather than as a percentage, since irregular geometries make percentage-based comparison less meaningful.

Sample Conditioning Before Testing

Accurate water absorption measurement requires a known dry reference mass. Specimens are dried in an oven under conditions appropriate to the material, for example 50 °C under vacuum for several hours or a higher temperature for heat-stable polymers, until constant mass is achieved (typically when successive weighings differ by less than 0.1 mg). After drying, specimens are cooled to room temperature in a desiccator and weighed immediately to establish the initial dry mass.

Testing Items

Depending on the material specification and end-use requirement, the following test items are commonly evaluated by accredited third-party laboratories and in-house quality departments:

• Water absorption after 24 hours of immersion in water at 23 °C (the most widely specified short-term indicator)
• Long-term immersion water absorption measured at extended intervals until saturation or equilibrium is reached
• Water absorption after immersion in boiling water, typically 30 minutes or 2 hours, used to accelerate comparison between materials
• Moisture absorption after exposure to 50 % relative humidity at 23 °C, which simulates atmospheric service conditions
• Water absorption expressed as mass change per unit surface area (mg/cm²) for specimens with variable or non-standard geometry
• Moisture absorption curves and apparent diffusion coefficients derived from periodic weighing, used for material modeling and prediction of in-service moisture content
• Supplementary dimensional measurements (swelling) before and after exposure, when dimensional stability is a design concern

Testing Methods

Immersion in Water at 23 °C (ISO 62 Method 1 / ASTM D570, 24-hour Procedure)

The conditioned and weighed specimen is fully immersed in distilled or deionized water maintained at 23 °C, with the entire surface in contact with water and specimens kept separated to avoid contact. After 24 hours, the specimen is removed, all surface water is removed immediately with a dry lint-free cloth, and the specimen is weighed to the nearest 1 mg. The mass gain is used to calculate the 24-hour water absorption value.

Immersion in Boiling Water (ISO 62 Methods 2 and 3)

To accelerate moisture uptake, specimens are immersed in boiling water for 30 minutes (Method 2) or 2 hours (Method 3). After the prescribed time, the specimen is transferred to water at room temperature for a defined cooling period (typically 15 ± 1 minutes), surface-dried, and weighed. Boiling immersion is particularly useful for comparing highly moisture-resistant engineering plastics or for screening materials intended for hot-water service.

Exposure to 50 % Relative Humidity (ISO 62 Method 4)

Specimens are placed in a conditioned atmosphere of 50 % relative humidity at 23 °C for a specified duration, commonly 24 hours or longer. This method characterizes absorption of water vapor from air rather than direct liquid contact and better reflects behavior in typical indoor or enclosed environments. It is especially relevant for hygroscopic materials such as polyamides, whose mechanical properties vary strongly with atmospheric moisture.

Long-Term Immersion and Saturation Determination

For saturation analysis, specimens remain immersed and are periodically removed, dried, weighed, and returned to the water. Weighing continues at increasing intervals until the mass change between successive measurements becomes negligible, indicating equilibrium moisture content. The resulting absorption-versus-time curve is characteristic of the material and thickness and follows Fickian diffusion behavior in many amorphous and semi-crystalline polymers.

Calculation and Expression of Results

The water absorption as a percentage of dry mass is calculated as: c = (m2 − m1) / m1 × 100, where m1 is the mass of the dry specimen and m2 is the mass after the exposure period. For specimens of non-standard geometry, the result may alternatively be expressed as mass change per unit surface area: cA = (m2 − m1) / A, where A is the total exposed surface area. Results must always be reported together with the specimen thickness, the exposure condition, and the test standard used, since all three strongly influence the value.

Factors Influencing Test Results

• Specimen thickness: thicker specimens absorb proportionally less water over a fixed short period because diffusion is time-limited
• Temperature: higher water temperature accelerates diffusion and increases equilibrium uptake in many polymers
• Crystallinity and orientation: higher crystallinity and molecular orientation reduce diffusion rates and equilibrium absorption
• Fillers and additives: hygroscopic fillers such as natural fibers or glass-fiber sizing chemistry can raise absorption, while well-bonded glass fibers typically lower the percentage by dilution
• Surface condition: machining marks, residual stress, and surface contamination alter uptake and must be controlled
• Drying completeness: incomplete drying before the test underestimates absorption by inflating the reference mass

Applications

Water absorption data are used throughout the plastics industry for material selection, product design, and quality assurance:

• Material comparison and selection for humid or submerged service, such as pump components, valve bodies, marine hardware, and appliance parts
• Electrical and electronics engineering, where moisture uptake correlates with reduced insulation resistance, dielectric strength, and surface tracking resistance in connectors, housings, and insulators
• Automotive engineering, particularly for under-hood components, sensor housings, and connectors exposed to splash water and high ambient humidity
• Polyamide component design, where equilibrium moisture content must be known to predict toughness, dimensional growth, and assembly tolerances
• Packaging development, where moisture barrier performance affects shelf life of moisture-sensitive contents
• Incoming material inspection and batch conformity verification against datasheet limits in ISO/IEC 17025-accredited third-party laboratories
• Research and development activities, including evaluation of new polymer grades, copolymer compositions, surface treatments, and compounding formulations
• Failure analysis, where unexpected moisture content is investigated as a possible cause of dimensional change, blistering during processing, or mechanical failure

FAQ

What is a typical water absorption value for common plastics?

Values vary widely by polymer family. Non-polar materials such as polyethylene, polypropylene, and PTFE typically show 24-hour absorption below 0.1 %. Engineering materials such as polycarbonate, POM, and PMMA generally fall between 0.1 % and 0.5 %. Polyamides are highly hygroscopic: PA6 may absorb around 1.3 % to 1.9 % in 24 hours and can exceed 8 % to 9 % at saturation, which is why conditioning requirements are so important for nylon parts.

What is the difference between ISO 62 and ASTM D570?

Both standards measure water absorption by gravimetric comparison before and after exposure, and their core immersion procedures are conceptually equivalent. Differences exist in preferred specimen dimensions, conditioning details, cooling procedures after boiling immersion, and reporting conventions. Because thickness and procedure strongly affect short-term results, values obtained under different standards should not be compared directly without confirming equivalence of specimen geometry and exposure conditions.

Why must specimens be dried before testing?

The calculation is based on the dry mass of the specimen. If the material already contains moisture from prior storage or molding, the reference mass is artificially high and the calculated absorption is underestimated. Drying to constant mass establishes a reproducible starting point and ensures comparable results between laboratories and production batches.

Why does the 24-hour value not represent the maximum water a plastic can absorb?

Water absorption is diffusion-controlled. In a 3 mm specimen, 24 hours is usually far too short for water to penetrate to the core, so the measured value reflects only partial saturation. Equilibrium or saturation absorption, determined through long-term immersion or humidity exposure, is required when designing for worst-case moisture conditions.

How does absorbed water affect plastic properties?

Absorbed water plasticizes the polymer, generally reducing tensile strength, flexural modulus, and hardness while increasing impact toughness in ductile materials such as polyamide. It causes dimensional swelling, can lower electrical insulation performance, and in bonded or coated assemblies may promote delamination, corrosion of inserts, or hydrolytic degradation at elevated temperatures.

How long does a standard water absorption test take?

A routine 24-hour immersion test, including drying and conditioning, typically requires two to three working days. Boiling immersion tests are shorter in exposure but still require the same drying and weighing steps. Saturation and diffusion studies may extend over several weeks, with periodic weighings until equilibrium is confirmed.